1. Field of the Invention
The present invention relates to exhaust gas purifying systems of an internal combustion engine, and more particularly to the exhaust gas purifying systems of a type that can effectively purify NOx contained in oxygen-richer exhaust gas from the engine.
2. Description of the Prior Art
As is known, diesel engines and lean-burn type gasoline engines are able to exhibit a satisfied fuel consumption. However, these engines belong to a group in which purification of NOx in oxygen-richer exhaust gas is rather difficult.
For dealing with such NOx purification, Japanese Patent Second Provisional Publication 2881262 discloses an exhaust gas purifying system that has a NOx-absorbing converter installed in an exhaust passage extending from the engine. That is, when the air/fuel ratio of the exhaust gas is higher than stoichiometric (that is, leaner air/fuel ratio), the NOx-absorbing converter (more specifically, the catalyzer in the converter) absorbs NOx in the exhaust gas, while when the air/fuel ratio of the exhaust gas is lower than stoichiometric (that is, richer air/fuel ratio), the NOx-absorbing converter releases NOx therefrom allowing reduction of the same in the exhaust gas.
The publication describes the characteristics of the NOx-absorbing agent used in the NOx-absorbing converter. That is, in order to effectively absorb NOx through oxidation absorption mechanism, the publication describes a need of increasing the basicity of the NOx-absorbing agent. The publication further describes that when the basicity is increased, the reducing ability of the agent becomes lowered, which would cause discharge of NOx to the open air without reducing the same. In view of this contradiction, the publication proposes a series arrangement of two, viz., upstream and downstream NOx-absorbing catalyzer units in the exhaust passage. That is, the two NOx-absorbing catalyzer units are installed in tandem in a common converter casing, having the reducing ability of the downstream catalyzer unit greater than that of the upstream catalyzer unit. In other words, the basicity of the NOx-absorbing agent in the downstream catalyzer unit is set lower than that in the upstream catalyzer unit.
However, the above-mentioned series arrangement of the two NOx-absorbing catalyzer units fails to exhibit a satisfied NOx purifying performance throughout a sufficiently widen temperature range of the exhaust gas.
In view of the above, the applicants have carried out various examinations and tests and finally found that the basicity of the NOx-absorbing agent has a certain connection with the temperature at which the NOx-absorbing agent exhibits the maximum NOx-absorbing performance. That is, for example, if the basicity is increased, the agent can exhibit a satisfied NOx-absorbing performance in a wider temperature range from low to high temperature, while, if the basicity is weak, the agent exhibits such performance only in a lower temperature range.
In the above-mentioned series arrangement, the two NOx-absorbing catalyzer units are installed in the common converter casing. This means that under operation of the engine, the two NOx-absorbing catalyzer units having different temperature characteristics are inevitably exposed to the same temperature environment.
In practical use, the converter casing is mounted on an exhaust tube at a position remote from the engine. Thus, when, due to lower-speed operation, lean-combustion operation or the like, the engine produces exhaust gas whose temperature is not sufficiently high, both the two NOx-absorbing catalyzer units fail to be heated up to their activation temperature (viz., light on temperature) for the NOx absorption. While, if the converter casing is arranged just downstream of the exhaust manifold of the engine, both the two NOx-absorbing catalyzer units are forced to introduce high temperature exhaust gas, and thus, at least the downstream NOx-absorbing catalyzer unit can not exhibit a satisfied absorbing performance against NOx. That is, as is mentioned above, the agent in the downstream NOx-absorbing catalyzer unit has a lower basicity exhibiting the NOx-absorbing performance only in a lower temperature range.
Besides the above, in the exhaust gas from the engine, there are contained sulfur contents (S). Thus, under operation of the engine, sulfur oxides (SOx) are also absorbed (or trapped) by the NOx-absorbing catalyzer units together with NOx in a similar absorption mechanism, which of course lowers the NOx-absorbing performance of the catalyzer units. In order to eliminate such sulfur poisoning (S-poisoning), it is necessary to heat up the NOx-absorbing agent to a certain releasing temperature. However, when, as is described above, the NOx-absorbing catalyzer units are arranged in a position where they can not introduce a sufficiently heated exhaust gas, releasing of the sulfur poisoning is impossible due to lack of the releasing temperature. Particularly, the NOx-absorbing agent with higher basicity tends to have a higher releasing temperature against sulfur poisoning, as compared with NOx-absorbing agent with lower basicity. Thus, in this case, releasing of sulfur poisoning from the NOx-absorbing agent having higher basicity is much difficult.
It is therefore an object of the present invention to provide an exhaust gas purifying system which is free of the above-mentioned shortcomings.
According to a first aspect of the present invention, there is provided an exhaust gas purification apparatus for an engine, which comprises a first NOx catalyst, disposed in a vicinity of an exhaust manifold of the engine, that traps NOx in a lean air-fuel ratio condition and releases trapped NOx in a stoichiometric or rich air-fuel ratio condition; a second NOx catalyst, disposed in a downstream side of the first NOx catalyst, that traps NOx in a lean air-fuel ratio condition and releases trapped NOx in a stoichiometric or rich air-fuel ratio condition, the second NOx catalyst showing a low basicity ability relative to the first NOx catalyst; and an exhaust passage that communicates the first NOx catalyst with the second NOx catalyst so that the second NOx catalyst is apart from the first NOx catalyst and that the second NOx catalyst is placed in a low temperature environment relative to the first NOx catalyst.
According to a second aspect of the present invention, there is provided an exhaust gas purification apparatus for an engine, which comprises a first NOx catalyst, disposed in a vicinity of an exhaust manifold of the engine, that traps NOx in a lean air-fuel ratio condition and releases trapped NOx in a stoichiometric or rich air-fuel ratio condition; a second NOx catalyst, disposed in a downstream side of the first NOx catalyst, that traps NOx in a lean air-fuel ratio condition and releases trapped NOx in a stoichiometric or rich air-fuel ratio condition, the second NOx catalyst showing a high reducing ability relative to the first NOx catalyst; and an exhaust passage that communicates the first NOx catalyst with the second NOx catalyst so that the second NOx catalyst is apart from the first NOx catalyst and that the second NOx catalyst is placed in a low temperature environment relative to the first NOx catalyst.
According to a third aspect of the present invention, there is provided an exhaust gas purifying system of an internal combustion engine, which comprises an exhaust passage extending from the engine through an exhaust manifold; a first NOx-trapping catalyzer unit disposed on an upstream part of the exhaust passage; a second NOx-trapping catalyzer unit disposed on a downstream part of the exhaust passage; and a structure that causes the first NOx-trapping catalyzer unit to be exposed to a relatively high temperature environment and causes the second NOx-trapping catalyzer unit to be exposed to a relatively low temperature environment, wherein each of the first and second NOx-trapping catalyzer units traps NOx when the exhaust gas applied thereto shows a leaner air/fuel ratio and releases and reduces NOx when the exhaust gas applied thereto shows a stoichiometric or richer air/fuel ratio, and wherein the first NOx-trapping catalyzer unit shows the basicity that is higher than that of the second NOx-trapping catalyzer unit.
According to a fourth aspect of the present invention, there is provided an exhaust gas purifying system of an internal combustion engine, which comprises an exhaust passage extending from the engine through an exhaust manifold; a first NOx-trapping catalyzer unit disposed on an upstream part of the exhaust passage; a second NOx-trapping catalyzer unit disposed on a downstream part of the exhaust passage; and a structure that causes the first NOx-trapping catalyzer unit to be exposed to a relatively high temperature environment and causes the second NOx-trapping catalyzer unit to be exposed to a relatively low temperature environment, wherein each of the first and second NOx-trapping catalyzer units traps NOx when the exhaust gas applied thereto shows a leaner air/fuel ratio and releases NOx when the exhaust gas applied thereto shows a stoichiometric or richer air/fuel ratio, and wherein the first NOx-trapping catalyzer unit has a low reducing ability as compared with the second NOx-trapping catalyzer unit, so that the first NOx-trapping catalyzer unit exhibits NOx-trapping performance to the extent of a high temperature as compared with the second NOx-trapping catalyzer unit.